1. Field of the Invention
This invention relates to an apparatus for measuring the weight of an object being carried by a fork lift truck.
2. Description of Prior Art
A typical fork truck has a telescopic mast comprising at least two sections of which the bottom section is mounted on the lift truck chassis. One mast section may be positioned relative to another by, for example, one or more hydraulic cylinders so as to vary the height of the mast between minimum and maximum limits. A vertically movable carriage is provided with rollers, or other guide means for engaging channels formed in the mast sections so as to contain carriage movement within the mast and support the moment created by the object being carried. The carriage is typically supported by two chains which pass over pulleys mounted near the top of the mast and are secured at one end to the carriage and at the other end to an anchorage point which is fixed to the bottom section of the mast. A fork or other means is mounted on the carriage and is used to engage the object for lifting and carrying.
Several methods are currently used for determining the weight of an object being carried by a fork lift truck. One method involves measuring the fluid pressure in the hydraulic rams or rams when the fork lift truck is supporting the load. The rams or rams must sustain the weight of the moveable mast section or sections, the carriage and the fork in addition to the weight of the load. The accuracy with which the weight of the object determined by this arrangement is limited because the measured fluid pressure is affected by the frictional forces present between the mast sections, as well as between the carriage and the mast. In addition, the proportion of the measured fluid pressure that is attributable to the object being weighed is in general small compared with the total fluid pressure. This requires a pressure transducer means with both a large pressure range and fine precision.
Another known arrangement involves measuring the tension in the chains(s) supporting the carriage. This is accomplished either by measuring the chain tension(s) directly, or more commonly by measuring the resulting compressive forces exerted between the supporting chain(s) and their anchorage point(s) on the mast. This method is capable of a greater accuracy than the first because the measurement is not dependent upon the weight of the mast sections or on the frictional forces between them. However, the measurement is affected by the frictional forces between the carriage and the mast which provides for a significant source of error. One scheme has been disclosed that improves the chain tension measurement by selecting a specific certain pulley diameter according the chain pitch, and another has been disclosed that utilizes an integrative technique while the carriage is being raised or lowered. In either case the weight determination according to this method is inherently inaccurate due to the frictional forces present between carriage and mast.
A more common method, but one that provides greater accuracy (typically <=0.10% of applied load) involves using four specially constructed dual circuit load cells. In this arrangement a dead frame is provided that affixes to the existing fork truck carriage. The load cells are attached to the dead frame and a new live frame, which has identical fork carrying features as the standard carriage, is attached to the load cells. All load forces and moments therefore pass through the load cell elements positioned between the live frame and the dead frame. The primary load cell outputs are then summed to generate a signal that is directly representative of the weight of the object being weighed.
Further, the load cells are equipped with a secondary sensing circuit that is sensitive to the load moment of the object being weighed and the resulting measurement is used to correct the primary weight signal making the scale substantially insensitive to the load movement. While this arrangement can be added on to an existing fork lift truck, because the device inherently moves the load carrying position forward from the original position, it has the distinct disadvantage of reducing the carrying capacity of the truck. In addition, because of their complexity the load cell sensors utilized in this method are less reliable and more expensive than single circuit load cell designs.
Another common method also provides a dead frame that affixes to the existing fork truck carriage and a new live frame, which as above has identical fork carrying features as the standard carriage, and is positioned somewhat forward of the of the dead frame. The live frame is mechanically connected to the dead frame by a series of flexural elements. These elements allow only vertical motion between the live frame and the dead frame, restricting all other degrees of freedom. The motion between the live frame and the dead frame is then restricted by one or more load cell sensors positioned between the two frames. The force that is measured is then directly representative of the object being weighed. In order to make the weight substantially independent of its position on the forks, the flexural elements are finely positioned so that the vertical forces in the flexural elements due to the load moment are effectively cancelled by one another.
In order to assure accuracy in this arrangement, the flexural elements must be designed so that they are significantly weak in the vertical direction. This allows for their effect on the weight measurement to be accounted for in the calibration but creates the inherent disadvantage of this method; the flexural elements must be substantial enough to carry the loads generated by the moment created by the object being weighed and yet have an insignificant effect in the vertical direction. Again, because this device moves the load position forward on the truck compared with the original load carrying position, the fork trucks carrying capacity is diminished when this apparatus is installed.